The field of pediatric exercise physiology is still an emerging science. Biological maturation and large variations in morphology of this group raise challenges in studying and describing physiologic responses in children and adolescents. Because of this variability, chronologic age is not a reliable means of comparison. Tanner staging has merit clinically, but not shown to be as reliable in exercise physiology. There is a lack of means to standardize measures for size and development. There are ethical aspects, which makes research in this population more challenging than its adult counterpart. Studies need to be appropriately designed with clear benefit outweighing risk in this vulnerable population. In fact, actual long-term risk of type of exercise may not be completely known.
The adolescent period appears to be unique because of the impact of the changing hormone milieu which is occurring. Prior to this period, the child's growth is largely driven by growth hormones. During pubertal growth, reproductive hormones come into play. This change affects not only growth and development, but physiologic responses as well (Figure 4-1).
Relationship between spurt in height, weight, muscle mass (fat free mass), fat (fat mass), testosterone levels, sexual maturity rating, and chronologic age.
A clear understanding of factors that promote growth itself is lacking. However, physical development of organ systems such as the cardiac, ventilatory, and musculoskeletal are considered the driving force of physiologic improvement of performance and capacity in children. Acute and chronic exercise may stimulate growth factors by impacting binding proteins and receptor sites. Growth factor variations with exercise may be associated with nutritional variation resulting in biological markers of overuse.
Increased sex hormones present in puberty leads to a variety of changes in physiologic functioning that impact exercise performance. Exercise training has been associated with an inhibition of hypothalamic–pituitary–gonadal axis. This impact of regular exercise on reproductive functioning may be mediated by nutritional state, caloric balance, body composition, or some combination of these. Regular vigorous exercise has been shown to slow linear growth in gymnasts only. While the significance of this inhibition of hormones is not completely understood, low estrogen levels may have a long-term effect on bone development.
Metabolic pathways in response to activity are thought to vary with development of children. Anaerobic glycolysis progressively rises as children age, whereas aerobic metabolic capacity declines. These changes progress steadily throughout childhood without significant influence of puberty. The relationship of this phenomenon to body size mimics the pattern seen in adults.1
Exercise responses in children are often compared to adults based on current understanding of adult exercise physiology. There are variations in responses between these two groups. Some of this variation is present despite normalization for size. A summary of physiologic responses for children is listed in Table 4-1.